<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* div {
            border: solid;
        } */
        .main {
            /* width: 800px;
            height: 800px; */
            position: fixed;
            top: 0;
            bottom: 0;
            left: 0;    
            right: 0;
        }
        .top{
            height: 100px;
            position: absolute;
            left: 0;
            right: 0;
            top: 0;
            background-color: aquamarine;
        }
        .bottom{
            height: 100px;
            position: absolute;
            left: 0;
            right: 0;
            bottom: 0;
            background-color: aquamarine;
        }
        .middle{
            position: absolute ;
            top: 100px;
            bottom: 100px;
            left: 0;    
            right: 0;
            background-color: aqua;
        }
    </style>
</head>
<body>
    <div class="main">
        <div class="top">top</div>
        <div class="middle">middle</div>
        <div class="bottom">bottom</div>
    </div>
</body>
</html>